It happens nearly a quarter-billion times a day in the USA: A car, truck or other vehicle is driven across one of the nation's 63,000 structurally compromised bridges.

That's from a new analysis by the industry group the American Road & Transportation Builders Association, which is warning that the situation might worsen.

The report comes against the backdrop of growing cries of alarm that the federal Highway Trust Fund, normally used to pay for roads and transit projects, could be insolvent by the fall unless Congress acts.

"Without congressional action, there will not be any federal support for any new road or bridge projects in any state in fiscal year 2015, which starts on Oct. 1," said Alison Black, chief economist for the road builders group.

Earlier this month, U.S. Sen. Patty Murray, a Democrat from Washington who chairs the Senate Appropriations subcommittee that handles transportation, warned her colleagues that the nation faces a "construction shutdown" later this year unless Congress acts. She called the potential problems with the Highway Trust Fund "another avoidable crisis," comparable to last fall's shutdown of the federal government.

Last week, the American Society of Civil Engineers urged Congress to take immediate action to avert insolvency in the trust fund. "We're at a critical crossroad," said president Randall Over.

The Highway Trust Fund is normally funded by revenue collected from the 18.4 cents-a-gallon federal gas tax. But the gas tax has not been increased since 1993, and soaring road-building costs have dwarfed receipts by as much as $20 billion a year in recent years.

Nearly one-tenth of the 607,380 bridges in the National Bridge Inventory, a database of information on bridges and tunnels, are rated as structurally deficient by the Federal Highway Administration; the average age of those bridges is 42 years. FHWA has estimated that the nation would need to spend about $20.5 billion a year to eliminate the backlog by 2028; the USA spends about $12.8 billion annually.

A structurally deficient bridge is not necessarily unsafe, according to the American Association of State Highway and Transportation Officials. When left open to traffic, these bridges typically require significant maintenance and repair to remain open, and eventually need to be rehabilitated or replaced to address deficiencies. To remain in service, these bridges are often posted with weight limits restricting the gross weight of vehicles permitted on the bridge.

The two-year, stop-gap funding bill passed by Congress in 2012 ends Sept. 30. Most infrastructure reform advocates are pushing for a 5- or 6-year funding bill of the sort that Congress has normally approved.

Options that have been discussed include raising the federal gas tax, implementing more tolling, supplementing gas tax revenue with money from closing corporate tax loopholes and transferring money from the General Fund.

David Hartgen, emeritus professor at the University of North Carolina-Charlotte and senior fellow at the libertarian Reason Foundation, says the nation has been making slow, but steady progress in reducing the backlog of deficient bridges.

"I would not use the word crisis," he said. "Things are a little bit more urgent than they have been in the past. I would say there is concern."